This invention relates to a solid-state laser oscillator which pumps a solid-state laser with a light output from a semiconductor laser to emit a laser beam therefrom.
Conventionally, in the field of this type of solid-state laser oscillator, a system wherein a laser beam output from a solid-state laser is modulated with an external modulator such as an acoustic optical element, and a system wherein modulation is made by modulating a semiconductor laser (called LD hereinafter) for pumping a solid-state laser and feeding a light output from the modulated LD to the solid-state laser are generally known as a system to modulate an output laser beam.
In order to shorten a rise time of a laser beam output from a solid-state laser based on the latter system, the following measures can be applied. In brief, two pieces of LD are arranged in a solid laser, and one of the two LDs is used to pump the solid-state laser to a threshold value or a value more than the threshold beforehand and a pulse pump light from the other LD which performs pulse pump according to a modulation signal is applied to the solid-state laser (This system is called bias pump system hereinafter).
In the aforesaid external modulation system, however, it is difficult to build a system having a compact configuration, which is one of the reasons for raising the system cost.
Also, in the aforesaid bias pump system, as a response speed of the solid-state laser is lower than a modulation breaking or interrupting speed in the LD, change of output from the solid laser can not follow change of output from the LD in the case of high speed modulation. For this reason, as shown by a character b.sub.1 (indicating output from the solid-state laser) in FIG. 10, a "relaxation oscillation" as described hereinafter occurs after outputs P.sub.1 and P.sub.2 from the solid-state laser, noises n.sub.1 and n.sub.2 are generated after fall of the outputs P.sub.1 and P.sub.2 from the solid-state laser. A character a.sub.1 (indicating a pulse output from the LD) shows a case where bias pump (for instance, 44 mW) is made first and pulse pump (for instance, 66 mW) is added to it. Because of the aforesaid noises n.sub.1 and n.sub.2, accurate modulation according to an external signal can not be made, and if this type of noise is generated, for instance, in a so-called laser printer during printing, bleeding or blur of outline of printed letters or figures occur.
Herein, the "relaxation oscillation" is generally defined as oscillation which occurs from a time when an external force is loaded to a system in balance and a new balanced state is established until a time when the aforesaid external force is removed to restore the original balanced state. A process where the "relaxation oscillation" in a laser beam occurs is as described below.
In other words, when a light output from an LD is constant, a population inversion density of a solid-state laser to which this output light is fed and a light density in the solid-state laser (a light output from the solid-state laser is in proportion to this light density) are also constant. Further, when output of light from the LD increases (for instance during rise of an LD pulse), a population inversion density in the solid-state laser increases and a light density in the solid-state laser increases. The aforesaid increase of a light density causes reduction of a population inversion density through increase of a stimulated emission rate which is proportional to the light density. Because of this reduction of population inversion density, the light density decreases, and the population inversion density again increases through decrease of the stimulated emission rate.
The repetition described above is continued at a constant cycle until output from the solid-state laser is stabilized, and this process is called "relaxation oscillation". The above-described process is also generated when output from an LD decreases at the time of fall of an LD pulse. In this case, however, the process is started from decrease of the population inversion density.